Method and Apparatus for Measuring a Parameter of a Lithographic Process, Substrate and Patterning Devices for use in the Method

ABSTRACT

A substrate has first and second target structures formed thereon by a lithographic process. Each target structure has two-dimensional periodic structure formed in a single material layer on a substrate using first and second lithographic steps, wherein, in the first target structure, features defined in the second lithographic step are displaced relative to features defined in the first lithographic step by a first bias amount that is close to one half of a spatial period of the features formed in the first lithographic step, and, in the second target structure, features defined in the second lithographic step are displaced relative to features defined in the first lithographic step by a second bias amount close to one half of said spatial period and different to the first bias amount. An angle-resolved scatter spectrum of the first target structure and an angle-resolved scatter spectrum of the second target structure is obtained, and a measurement of a parameter of a lithographic process is derived from the measurements using asymmetry found in the scatter spectra of the first and second target structures.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/237,246 filed Aug. 15, 2019, which claims benefit under 35 U.S.C §119(e) to U.S. Provisional Applications No. 62/210,938, filed on Aug.27, 2015 and U.S. Provisional Applications No. 62/301,880, filed on Mar.1, 2016, which are all incorporated by reference herein in theirentireties.

BACKGROUND Field of the Invention

The present invention relates to methods of manufacture of products suchas semiconductor devices using lithographic techniques.

Background Art

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.,including part of, one, or several dies) on a substrate (e.g., a siliconwafer). Multiple layers, each having a particular pattern and materialcomposition, are applied to define functional devices andinterconnections of the finished product.

Current and next generation processes often rely on so-called multiplepatterning techniques to produce device features having dimensions farsmaller than can be printed directly by the lithographic apparatus.Multiple patterning steps, each having its own mask or reticle, areperformed to define a desired device pattern in a single layer on thesubstrate. Many different examples of multiple patterning are known. Insome processes, a regular, grid structure is formed as a basis for thedesired device pattern. Then using a circuit-specific mask pattern,lines that form the grid structure are cut at specific locations toseparate the lines into individual segments. The grid structure may beexceptionally fine in dimensions, with a pitch in the tens or even teensof nanometers.

In a lithographic process, it is desirable frequently to makemeasurements of structures created, e.g., for process control andverification. Various tools for making such measurements are known,including scanning electron microscopes, which are often used to measurecritical dimension (CD), and specialized tools to measure overlay, theaccuracy of alignment of two layers of a substrate. Final performance ofmanufactured device depends critically on the accuracy of positioningand dimensioning of the cut mask relative to the grid structure. (Thecut mask in this context is what defines the circuit-specific locationsat which the grid structure is modified to form functional circuits.)Overlay error may cause cutting or other modification to occur in awrong place. Dimensional (CD) errors may cause cuts be too large, or toosmall (in an extreme case, cutting a neighboring grid line by mistake,or failing to cut the intended grid line completely).

Other performance parameters of the lithographic process may be also ofinterest, for example in optical lithography parameters of focus andexposure dose may also require measuring.

However, the dimensions of modern product structures are so small thatthey cannot be imaged by optical metrology techniques. Small featuresinclude for example those formed by multiple patterning processes, andpitch-multiplication. (These terms are explained further below.) Ineffect, the structures are too small for traditional metrologytechniques which cannot “see” them. Hence, targets used for high-volumemetrology often use features that are much larger than the productswhose overlay errors or critical dimensions are the property ofinterest.

While scanning electron microscopes are able to resolve modern productsstructures, measurements performed with scanning electron microscopesare much more time consuming, as well as more expensive, than opticalmeasurements.

SUMMARY

The inventors have recognized that it is possible to perform metrologymeasurements on structures with dimensions and processing similar toproduct structures, by using zeroth order light scattered by thesestructures.

In a first aspect of the invention, there is provided a method ofmeasuring a parameter of a lithographic process, the lithographicprocess being for forming a two-dimensional, periodic product structurein a single material layer using two or more lithographic steps, themethod comprising: providing first and second target structures, eachtarget structure comprising a two-dimensional periodic structure formedin a single material layer on a substrate using first and secondlithographic steps, wherein, in the first target structure, featuresdefined in the second lithographic step are displaced relative tofeatures defined in the first lithographic step by a first bias amountthat is close to one half of a spatial period of the features formed inthe first lithographic step, and, in the second target structure,features defined in the second lithographic step are displaced relativeto features defined in the first lithographic step by a second biasamount close to one half of said spatial period and different to thefirst bias amount; obtaining an angle-resolved scatter spectrum of thefirst target structure and an angle-resolved scatter spectrum of thesecond target structure; and deriving a measurement of said parameterusing asymmetry found in the scatter spectrum of the first targetstructure and asymmetry found in the scatter spectrum of the secondtarget structure.

In some embodiments, obtaining the angle-resolved scatter spectrum ofeach target structure comprises: illuminating the target structure withradiation; and detecting the angle-resolved scatter spectrum using zeroorder radiation scattered by the target structure.

The spatial period of each target structure is significantly shorterthan a wavelength of the radiation used to illuminate the targetstructures.

The method may further comprise selecting the wavelength of radiationfrom a range of available wavelengths so as to optimize strength andlinearity of asymmetry in the angle-resolved scatter spectra of thetarget structures.

In some embodiments, the step of deriving said parameter comprisescalculating a measurement of overlay error relating to said productstructures using the asymmetry found in the scatter spectrum of thefirst target structure, the asymmetry found in the scatter spectrum ofthe second target structure and knowledge of the first bias amount andthe second bias amount.

Features of the target structures that are defined in the firstlithographic step may comprise a grid structure defining said spatialperiod in a first direction, and features of said target structures thatare defined in the second lithographic step may comprise modificationsof the grid structure at locations spaced periodically in atwo-dimensional periodic arrangement.

The features of said target structures that are defined in the firstlithographic step may further comprise a grid structure defining saidspatial period in a first direction, and features of said targetstructures that are defined in the second lithographic step may furthercomprise cuts in elements of the grid structure.

In some embodiments, the first target structure and the second targetstructure may be formed by etching and/or deposition processes after thefirst and second lithographic steps have been used to define theirfeatures

In some embodiments, a product structure may be formed in the samematerial layer elsewhere on the same substrate using said first andsecond lithographic steps, wherein, in the product structure, featuresdefined in the second lithographic step are not displaced relative tofeatures defined in the first lithographic step by any bias amount.

The invention further provides a substrate for use in measuring aparameter of a lithographic process, the substrate comprising first andsecond target structures, each target structure comprising atwo-dimensional periodic structure formed in a single material layerusing said first and second lithographic steps, wherein, in the firsttarget structure, features defined in the second lithographic step aredisplaced relative to features defined in the first lithographic step bya first bias amount that is close to one half of a spatial period of thefeatures formed in the first lithographic step, and, in the secondtarget structure, features defined in the second lithographic step aredisplaced relative to features defined in the first lithographic step bya second bias amount that is close to one half of said spatial periodand different to the first bias amount.

The invention yet further provides a metrology apparatus for use in amethod according to the invention as set forth above.

In some embodiments, the metrology apparatus may comprise: a support fora substrate on which a first target structure and a second targetstructure have been formed; an optical system for selectivelyilluminating each target structure with radiation and collecting atleast zero order radiation scattered by the target structure; a detectorfor detecting an angle-resolved scatter spectrum of each using said zeroorder radiation; and a processor arranged to derive a parameter of alithographic process using asymmetry of the angle-resolved scatterspectrum of the first target structure and asymmetry of theangle-resolved scatter spectrum of the second target structure.

The invention yet further provides a lithographic system comprising: alithographic apparatus for use in a lithographic process; and ametrology apparatus according to the invention as set forth above foruse in measuring a parameter of the lithographic process using first andsecond target structures formed at least partially using thelithographic apparatus.

The invention yet further provides a computer program product comprisingmachine readable instructions which, when run on a suitable processor,cause the processor to perform the deriving step of the method accordingto the invention as set forth above.

The invention yet further provides a method to determine an overlayerror on a substrate on which product structures have been formed, theproduct structures including first product features that have beendefined by a first lithographic process and second product features thathave been defined by a second lithographic process, the overlay errorcomprising a positional deviation between the first product features andthe second product features, the method comprising: providing a firsttarget structure on the substrate, the first target structure comprisingfirst target features defined by the first lithographic process andsecond target features defined by the second lithographic step, apositional relationship between the first target features and the secondtarget features depending on a first bias value and the overlay error;and providing a second target structure on the substrate, the secondtarget structure comprising third target features defined by the firstlithographic process and fourth target features defined by the secondlithographic step, a positional relationship between the third targetfeatures and the fourth target features depending on a second bias valueand the overlay error; detecting a first angle-resolved scatter spectrumusing zero order radiation diffracted from the first target structure;detecting a second angle-resolved scatter spectrum using zero orderradiation diffracted from the second target structure; calculating ameasurement of the overlay error based on asymmetry observed in thefirst angle-resolved scatter spectrum and the second angle-resolvedscatter spectrum and on knowledge of the first bias value and the secondbias value.

Further aspects, features and advantages of the invention, as well asthe structure and operation of various embodiments of the invention, aredescribed in detail below with reference to the accompanying drawings.It is noted that the invention is not limited to the specificembodiments described herein. Such embodiments are presented herein forillustrative purposes only. Additional embodiments will be apparent topersons skilled in the relevant art(s) based on the teachings containedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example,with reference to the accompanying drawings in which:

FIG. 1 depicts a lithographic apparatus together with other apparatusesforming a production facility for semiconductor devices;

FIG. 2 depicts a scatterometer configured to capture an angle-resolvedscatter spectrum according to an embodiment of the present invention;

FIG. 3 illustrates a target structure according to a first embodiment ofthe present invention;

FIG. 4 schematically illustrates part of a set of patterning devicesused applying patterns to a substrate in the formation of the targetstructures of FIG. 3;

FIGS. 5(a)-5(c) schematically illustrate stages in a known multiplepatterning process;

FIGS. 6(a)-6(c) illustrate stages in forming first and second targetstructures in a multiple patterning process according to an embodimentof the present invention;

FIG. 7 is a flowchart of a method for measuring a parameter of alithographic process according to an embodiment of the presentinvention;

FIG. 8 schematically illustrates the principles of a conventional methodof measuring overlay;

FIG. 9 illustrates simulated variation of asymmetry against overlay, fordifferent wavelengths of radiation scattered from the example targetstructure; and

FIG. 10 shows simulated pupil images of asymmetry in scatter spectra ofan example target structure with different overlay values.

DETAILED DESCRIPTION

Before describing embodiments of the invention in detail, it isinstructive to present an example environment in which embodiments ofthe present invention may be implemented.

FIG. 1 at 200 shows a lithographic apparatus LA as part of an industrialproduction facility implementing a high-volume, lithographicmanufacturing process. In the present example, the manufacturing processis adapted for the manufacture of for semiconductor products (integratedcircuits) on substrates such as semiconductor wafers. The skilled personwill appreciate that a wide variety of products can be manufactured byprocessing different types of substrates in variants of this process.The production of semiconductor products is used purely as an examplewhich has great commercial significance today.

Within the lithographic apparatus (or “litho tool” 200 for short), ameasurement station MEA is shown at 202 and an exposure station EXP isshown at 204. A control unit LACU is shown at 206. In this example, eachsubstrate visits the measurement station and the exposure station tohave a pattern applied. In an optical lithographic apparatus, forexample, a projection system is used to transfer a product pattern froma patterning device MA onto the substrate using conditioned radiationand a projection system. This is done by forming an image of the patternin a layer of radiation-sensitive resist material.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. The patterning MA device maybe a mask or reticle, which imparts a pattern to a radiation beamtransmitted or reflected by the patterning device. Well-known modes ofoperation include a stepping mode and a scanning mode. As is well known,the projection system may cooperate with support and positioning systemsfor the substrate and the patterning device in a variety of ways toapply a desired pattern to many target portions across a substrate.Programmable patterning devices may be used instead of reticles having afixed pattern. The radiation for example may include electromagneticradiation in the deep ultraviolet (DUV) or extreme ultraviolet (EUV)wavebands. The present disclosure is also applicable to other types oflithographic process, for example imprint lithography and direct writinglithography, for example by electron beam.

The lithographic apparatus control unit LACU which controls all themovements and measurements of various actuators and sensors to receivesubstrates W and reticles MA and to implement the patterning operations.LACU also includes signal processing and data processing capacity toimplement desired calculations relevant to the operation of theapparatus. In practice, control unit LACU will be realized as a systemof many sub-units, each handling the real-time data acquisition,processing and control of a subsystem or component within the apparatus.

Before the pattern is applied to a substrate at the exposure stationEXP, the substrate is processed in at the measurement station MEA sothat various preparatory steps may be carried out. The preparatory stepsmay include mapping the surface height of the substrate using a levelsensor and measuring the position of alignment marks on the substrateusing an alignment sensor. The alignment marks are arranged nominally ina regular grid pattern. However, due to inaccuracies in creating themarks and also due to deformations of the substrate that occurthroughout its processing, the marks deviate from the ideal grid.Consequently, in addition to measuring position and orientation of thesubstrate, the alignment sensor in practice must measure in detail thepositions of many marks across the substrate area, if the apparatus isto print product features at the correct locations with very highaccuracy. The apparatus may be of a so-called dual stage type which hastwo substrate tables, each with a positioning system controlled by thecontrol unit LACU. While one substrate on one substrate table is beingexposed at the exposure station EXP, another substrate can be loadedonto the other substrate table at the measurement station MEA so thatvarious preparatory steps may be carried out. The measurement ofalignment marks is therefore very time-consuming and the provision oftwo substrate tables enables a substantial increase in the throughput ofthe apparatus. If the position sensor IF is not capable of measuring theposition of the substrate table while it is at the measurement stationas well as at the exposure station, a second position sensor may beprovided to enable the positions of the substrate table to be tracked atboth stations. Lithographic apparatus LA may for example is of aso-called dual stage type which has two substrate tables and twostations—an exposure station and a measurement station—between which thesubstrate tables can be exchanged.

Within the production facility, apparatus 200 forms part of a “lithocell” or “litho cluster” that contains also a coating apparatus 208 forapplying photosensitive resist and other coatings to substrates W forpatterning by the apparatus 200. At an output side of apparatus 200, abaking apparatus 210 and developing apparatus 212 are provided fordeveloping the exposed pattern into a physical resist pattern. Betweenall of these apparatuses, substrate handling systems take care ofsupporting the substrates and transferring them from one piece ofapparatus to the next. These apparatuses, which are often collectivelyreferred to as the track, are under the control of a track control unitwhich is itself controlled by a supervisory control system SCS, whichalso controls the lithographic apparatus via lithographic apparatuscontrol unit LACU. Thus, the different apparatus can be operated tomaximize throughput and processing efficiency. Supervisory controlsystem SCS receives recipe information R which provides in great detaila definition of the steps to be performed to create each patternedsubstrate.

Once the pattern has been applied and developed in the litho cell,patterned substrates 220 are transferred to other processing apparatusessuch as are illustrated at 222, 224, 226. A wide range of processingsteps is implemented by various apparatuses in a typical manufacturingfacility. For the sake of example, apparatus 222 in this embodiment isan etching station, and apparatus 224 performs a post-etch annealingstep. Further physical and/or chemical processing steps are applied infurther apparatuses, 226, etc. Numerous types of operation can berequired to make a real device, such as deposition of material,modification of surface material characteristics (oxidation, doping, ionimplantation etc.), chemical-mechanical polishing (CMP), and so forth.The apparatus 226 may, in practice, represent a series of differentprocessing steps performed in one or more apparatuses. As anotherexample, apparatus and processing steps may be provided for theimplementation of self-aligned multiple patterning, to produce multiplesmaller features based on a precursor pattern laid down by thelithographic apparatus.

As is well known, the manufacture of semiconductor devices involves manyrepetitions of such processing, to build up device structures withappropriate materials and patterns, layer-by-layer on the substrate.Accordingly, substrates 230 arriving at the litho cluster may be newlyprepared substrates, or they may be substrates that have been processedpreviously in this cluster or in another apparatus entirely. Similarly,depending on the required processing, substrates 232 on leavingapparatus 226 may be returned for a subsequent patterning operation inthe same litho cluster, they may be destined for patterning operationsin a different cluster, or they may be finished products to be sent fordicing and packaging.

Each layer of the product structure requires a different set of processsteps, and the apparatuses 226 used at each layer may be completelydifferent in type. Further, even where the processing steps to beapplied by the apparatus 226 are nominally the same, in a largefacility, there may be several supposedly identical machines working inparallel to perform the step 226 on different substrates. Smalldifferences in set-up or faults between these machines can mean thatthey influence different substrates in different ways. Even steps thatare relatively common to each layer, such as etching (apparatus 222) maybe implemented by several etching apparatuses that are nominallyidentical but working in parallel to maximize throughput. In practice,moreover, different layers require different etch processes, for examplechemical etches, plasma etches, according to the details of the materialto be etched, and special requirements such as, for example, anisotropicetching.

The previous and/or subsequent processes may be performed in otherlithography apparatuses, as just mentioned, and may even be performed indifferent types of lithography apparatus. For example, some layers inthe device manufacturing process which are very demanding in parameterssuch as resolution and overlay may be performed in a more advancedlithography tool than other layers that are less demanding. Thereforesome layers may be exposed in an immersion type lithography tool, whileothers are exposed in a ‘dry’ tool. Some layers may be exposed in a toolworking at DUV wavelengths, while others are exposed using EUVwavelength radiation.

In order that the substrates that are exposed by the lithographicapparatus are exposed correctly and consistently, it is desirable toinspect exposed substrates to measure properties such as overlay errorsbetween subsequent layers, line thicknesses, critical dimensions (CD),etc. Accordingly a manufacturing facility in which litho cell LC islocated also includes metrology system which receives some or all of thesubstrates W that have been processed in the litho cell. Metrologyresults are provided directly or indirectly to the supervisory controlsystem SCS. If errors are detected, adjustments may be made to exposuresof subsequent substrates, especially if the metrology can be done soonand fast enough that other substrates of the same batch are still to beexposed. Also, already exposed substrates may be stripped and reworkedto improve yield, or discarded, thereby avoiding performing furtherprocessing on substrates that are known to be faulty. In a case whereonly some target portions of a substrate are faulty, further exposurescan be performed only on those target portions which are good.

Also shown in FIG. 1 is a metrology apparatus 240 which is provided formaking measurements of parameters of the products at desired stages inthe manufacturing process. A common example of a metrology station in amodern lithographic production facility is a scatterometer, for examplean angle-resolved scatterometer or a spectroscopic scatterometer, and itmay be applied to measure properties of the developed substrates at 220prior to etching in the apparatus 222. Using metrology apparatus 240, itmay be determined, for example, that important performance parameterssuch as overlay or critical dimension (CD) do not meet specifiedaccuracy requirements in the developed resist. Prior to the etchingstep, the opportunity exists to strip the developed resist and reprocessthe substrates 220 through the litho cluster. The metrology results 242from the apparatus 240 can be used to maintain accurate performance ofthe patterning operations in the litho cluster, by supervisory controlsystem SCS and/or control unit LACU 206 making small adjustments overtime, thereby minimizing the risk of products being madeout-of-specification, and requiring re-work.

Additionally, metrology apparatus 240 and/or other metrology apparatuses(not shown) can be applied to measure properties of the processedsubstrates 232, 234, and incoming substrates 230. The metrologyapparatus can be used on the processed substrate to determine importantparameters such as overlay or CD. In accordance with embodiments of thepresent disclosure, the metrology apparatus is used to measureproperties of structures having the same material and dimensions asfunctional product structures, which have been formed using one or morelithographic steps, etching and other processes after lithographicexposure.

FIG. 2 shows the basic elements of a known angle-resolved scatterometerthat may be used as a metrology apparatus in embodiments of the presentdisclosure. In this type of metrology apparatus, radiation emitted by aradiation source 11 is conditioned by an illumination system 12. Forexample, illumination system 12 may include a collimating using lenssystem 12 a, a color filter 12 b, a polarizer 12 c and an aperturedevice 13. The conditioned radiation follows an illumination path IP, inwhich it is reflected by partially reflecting surface 15 and focusedinto a spot S on substrate W via a microscope objective lens 16. Ametrology target T may be formed on substrate W. Lens 16, has a highnumerical aperture (NA), for example at least 0.9 or at least 0.95.Immersion fluid can be used to obtain with numerical apertures greaterthan 1, if desired.

As in the lithographic apparatus LA, one or more substrate tables may beprovided to hold the substrate W during measurement. Coarse and finepositioners may be configured to accurately position the substrate inrelation to a measurement optical system. Various sensors and actuatorsare provided for example to acquire the position of a target ofinterest, and to bring it into position under the objective lens 16.Typically many measurements will be made on targets at differentlocations across substrate W. The substrate support can be moved in Xand/or Y directions to acquire different targets, and in the Z directionto obtain a desired focusing of the optical system on the target. It isconvenient to think and describe operations as if the objective lens andoptical system being brought to different locations on the substrate,when in practice the optical system may remain substantially stationaryand only the substrate moves. In other apparatuses, relative movement inone direction is implemented by physical movement of the substrate,while relative movement in orthogonal direction is implemented byphysical movement of the optical system. Provided the relative positionof the substrate and the optical system is correct, it does not matterin principle whether one or both of those is moving in the real world.

When the radiation beam is incident on the beam splitter 16 part of itis transmitted through the beam splitter (partially reflecting surface15) and follows a reference path RP towards a reference mirror 14.

Radiation reflected by the substrate, including radiation diffracted byany metrology target T, is collected by lens 16 and follows a collectionpath CP in which it passes through partially reflecting surface 15 intoa detector 19. The detector may be located in the back-projected pupilplane P, which is at the focal length F of the lens 16. In practice, thepupil plane itself may be inaccessible, and may instead be re-imagedwith auxiliary optics (not shown) onto the detector located in aso-called conjugate pupil plane P′. The detector may be atwo-dimensional detector so that a two-dimensional angular scatterspectrum or diffraction spectrum of a substrate target 30 can bemeasured. In the pupil plane or conjugate pupil plane, the radialposition of radiation defines the angle of incidence/departure of theradiation in the plane of focused spot S, and the angular positionaround an optical axis O defines azimuth angle of the radiation. Thedetector 19 may be, for example, an array of CCD or CMOS sensors, andmay use an integration time of, for example, 40 milliseconds per frame.

Radiation in reference path RP is projected onto a different part of thesame detector 19 or alternatively on to a different detector (notshown). A reference beam is often used for example to measure theintensity of the incident radiation, to allow normalization of theintensity values measured in the scatter spectrum.

The various components of illumination system 12 can be adjustable toimplement different metrology ‘recipes’ within the same apparatus. Colorfilter 12 b may be implemented for example by a set of interferencefilters to select different wavelengths of interest in the range of,say, 405-790 nm or even lower, such as 200-300 nm. An interferencefilter may be tunable rather than comprising a set of different filters.A grating could be used instead of interference filters. Polarizer 12 cmay be rotatable or swappable so as to implement different polarizationstates in the radiation spot S. Aperture device 13 can be adjusted toimplement different illumination profiles. Aperture device 13 is locatedin a plane P″ conjugate with pupil plane P of objective lens 16 and theplane of the detector 19. In this way, an illumination profile definedby the aperture device defines the angular distribution of lightincident on substrate radiation passing through different locations onaperture device 13.

The detector 19 may measure the intensity of scattered light at a singlewavelength (or narrow wavelength range), or it may measure the intensityseparately at multiple wavelengths, or integrated over a wavelengthrange. Furthermore, the detector may separately measure the intensity oftransverse magnetic- and transverse electric-polarized light and/or thephase difference between the transverse magnetic-polarized light andtransverse electric-polarized light.

In the known angle-resolved scatterometer represented schematically inFIG. 2, a metrology target T is provided on substrate W. Formeasurements, this target may comprise a 1-D grating, which is printedsuch that after development, it is an array of solid resist lines.Alternatively, the target may be a 2-D grating, which is printed suchthat after development, the grating is formed of solid resist pillars orvias (contact holes) in the resist. The bars, pillars or vias mayalternatively be etched into the substrate. Measurements of parameterssuch as line widths and shapes, may be obtained by an iterativereconstruction process, performed by processing unit PU, from knowledgeof the printing step and/or other scatterometry processes.

In addition to measurement of parameters by reconstruction,angle-resolved scatterometry is useful in the measurement of asymmetryof features in product and/or resist patterns. A particular applicationof asymmetry measurement is for the measurement of overlay, where thetarget comprises one set of periodic features superimposed on another.The concepts of asymmetry measurement using the instrument of FIG. 2 aredescribed for example in published patent application US2006066855A1cited above. Simply stated, while the positions of the higherdiffraction orders (1^(st) order and above) in the diffraction spectrumof a periodic target are determined only by the periodicity of thetarget, asymmetry of intensity levels in the diffraction spectrum isindicative of asymmetry in the individual features which make up thetarget. In the instrument of FIG. 2, where detector 19 may be an imagesensor, such asymmetry in the higher diffraction orders appears directlyas asymmetry in the pupil image recorded by detector 19. This asymmetrycan be measured by digital image processing in unit PU, and calibratedagainst known values of overlay.

For very fine product structures having features many times smaller thanthe wavelength of the illuminating radiation, however, higher orderdiffraction signals are not captured by collection path CP of theoptical system. Accordingly, conventional methods of diffraction-basedoverlay measurement are not able to reveal the type of overlay errorsthat may cause performance issues in very fine product structures formedby a modern multiple-patterning process.

FIG. 3 shows a measurement target 30 formed on a substrate W accordingto an embodiment of the present disclosure. The measurement targetcomprises a first target structure 31 and a second target structure 32.Examples of these will be described in greater detail below withreference to FIG. 6. Both the first target structure and the secondtarget structure are comprised of features which have dimensions similarto those of product features. The first target structure and secondtarget structure may be formed in the same material layer as productfeatures formed on the same substrate, and may be formed by the sameprocesses as the product features. For example, the first targetstructure and the second target structure may be formed in a singlelayer by multiple patterning steps. In another example, the first targetstructure and the second target structure are formed by the same etchingsteps as product structures on the substrate. Such product structuresmay be formed elsewhere on the same substrate, or it may be a substratededicated to carrying metrology targets only. In this regard, thesubstrate W in this example may be one of the substrates 232 or 234 inthe process illustrated in FIG. 1, rather than one of the substrates 220that have not yet been etched.

In the example, both the first target structure and the second targetstructure are two-dimensional structures, with periodicity in at leastsome features along one or both of the X direction or the Y direction.Whatever the periodicity of the structure as a whole, features withinthe structure are arrayed in a first direction (e.g. the X direction)with a pitch (spatial period) similar to that of product features to beformed by the lithographic process under investigation. Each targetstructure as a whole may be periodic in one or more directions.

Whatever the periodicity of the structure as a whole, it is atwo-dimensional structure in the sense that it has features varying inboth the X direction and the Y direction. By comparison, a“one-dimensional” grating structure may extend in two dimensions over anarea of a substrate, but (at least within the illumination spot S of themetrology apparatus) varies only in one direction. In other words,references to two-dimensional structures in the present disclosure canbe interpreted so that each target structure comprises features havingnon-zero components in a complementary Fourier space in both k_(x) andk_(y) directions (k are wavenumbers).

As can be seen, the measurement target 30 in this example has a set ofdimensions that are bigger than the irradiation spot S of the metrologyapparatus. This is also known as “underfilling” the target, and avoidsinterference of other structures in the obtained signals. For example,the target may be 40×40 μm or larger. With an appropriate illuminationsystem, it is possible to reduce the size of the irradiation spot. Thiswould enable a commensurate reduction of the size of the target, forexample as small as 10×10 μm. Reducing the size of a measurement targetis important as it enables the target to be placed within product areason a substrate without using excessive amounts of substrate real estate,which can otherwise be used for product structures.

In this example, the targets structures 31, 32 are each periodic in botha first (X) direction and a second (Y) direction. The first targetstructure 31 and the second target structure 32 are in one embodimentdefined by of a first set of features and a second set of features. Inone embodiment, represented schematically in FIG. 3, the first featurescomprise a plurality of linear elements defined by a first lithographicstep, the linear elements being arranged in a periodic arrangement. Inthis embodiment, the plurality of linear elements are modified by thesecond set of features to form a two-dimensional periodic structure.Specifically, the second set of features, comprises a periodicarrangement of locations where portions of the linear elements have beenremoved. The locations of these “cuts” are defined by the secondlithographic process, and have a two-dimensional periodic arrangement.Typically the shortest pitch (highest spatial frequency) of all thespatial frequency components will be that of the grid formed using thefirst lithographic step. The grid may be one-dimensional ortwo-dimensional. The two-dimensional structure comprising a grid oflinear elements with cuts is similar in its spatial frequency componentsto product structures that are to be produced on the same or anothersubstrate using the same lithographic process.

FIG. 4 shows schematically the overall layout of a first patterningdevice MA1, such as a reticle. The patterning device MA1 may comprisefeatures 400 defining a number of metrology targets and functionalproduct pattern areas 402. As is well known, patterning device M maycontain a single product pattern, or an array of product patterns if thefield of the lithographic apparatus is large enough to accommodate them.The example in FIG. 4 shows four product areas labeled D1 to D4. Targetfeatures 400 are placed in scribe lane areas adjacent these devicepattern areas and between them. The substrate W will eventually be dicedinto individual products by cutting along these scribe lanes, so thatthe presence of the targets does not reduce the area available forfunctional product structures. Where targets are small enough, they mayalso be deployed within the product areas 402, to allow closermonitoring of lithography and process performance across the substrate.Some in-die target features 404 of this type are shown in product areasD1-D4.

While FIG. 4 shows the patterning devices MA1 the same pattern isreproduced on the substrate W after the first lithographic step, andconsequently the above description applies to the substrate W as well asthe patterning device. Often a feature on the substrate will be defineddirectly by corresponding features on the patterning device. As is alsoknown, however, the relationship between the pattern on the patterningdevice and the finished features on the substrate is more complex. Thiscan be especially so when techniques such as pitch multiplication andmultiple patterning are applied in the processes described here.

Additionally, a second patterning device MA2 is shown in FIG. 4. Aseparate patterning device is needed for each lithographic step of thelithographic process. These patterning devices are just two among alarger set of patterning devices that will be used in a sequence oflithographic steps to make a finished product by the process illustratedin FIG. 1. In this example, the patterning devices MA1 and MA2 aredesigned to be used together in a multiple patterning process, so as todefine target structures and product structures within a single materiallayer.

Similarly to the first patterning device, the second patterning devicecomprises a number of metrology target features 400′ and a number offunctional product areas 402′. The layout is very similar between thetwo patterning devices at a macroscopic level, but at the microscopiclevel the patterns may be very different. Thus, the second patterningdevice may define new features of the target structures and/orfunctional product patterns, to be added to features defined in thefirst lithographic step. Alternatively, or in addition, the secondpatterning device may define features which modify the features definedin a first lithographic step. As an example, the first patterning deviceMA1 may define (directly or indirectly) a grid of features that areformed on the substrate using a first lithographic process. The secondpatterning device MA2 may define a number of features which modifyelements of the grid structure, during a second lithographic process.

Referring now to FIG. 5, an example of multiple patterning to form aproduct structure on a substrate is illustrated. At (a), a first gridstructure can be seen that comprises a plurality of grid elements 510,512, 514, 516, 518, 520 that are arranged in a periodic arrangement in afirst direction. The features of the first grid structure have beendefined by a first patterning device MA1 in a first lithographic step.In an example, however, the grid structure is not defined directly bythe patterning on the first patterning device, but has been formed byusing pitch multiplication (e.g. doubling, quadrupling). Pitchmultiplication allows the production of structures having a far finerpitch than anything that can be formed directly using lithographicapparatus LA. It is of course to be noted that pitch multiplication ismerely one exemplary method for forming grid structures.

The next process step to form a functional device pattern by multiplepatterning typically involves local modification of some or all elementsof the grid structure. In the present example, modification involvesremoving material at selected locations along the elements of the firstgrid structure, so as to cut each grid element into a number ofindividual elements. In the finished product, the elements may forexample perform metallic conductors, connecting functional devices andother conductors formed in layers above and/or below the layer shown.Other types of modification may be envisaged in principle, and cuttingwill be used as an illustration in the following description, onlybecause it is the most common example of modification. Also,modification of the elements should be understood as one example ofmodification of the first gird structure generally. Modification of thefirst grid structure could for example include locally bridging a gapbetween elements, rather than modifying the elements themselves. In thisway, the gaps between elements become divided into disconnected gaps,which may be useful in forming functional device structures insubsequent process steps.

To achieve the local cutting of the grid elements 510, 512, 514, 516,518, 520, a second lithographic process is performed using secondpatterning device MA2 to define a cut mask 522, illustrated by thedashed line in view (b). Cut mask 522 can be formed of photosensitiveresist material which substantially covers the first grid structure,except for small apertures 524. A patterning device (MA in FIG. 1) canbe provided with the appropriate pattern to form the cut mask aperturesdirectly by imaging in the resist, or indirectly in some way. As can beseen in view (b), small portions 526 of the grid elements are exposed inthe apertures 524. In the present example, the apertures 524 arearranged in a periodic manner in both the first and a second directionorthogonal to the first direction. The periodicity of the cut maskpattern is lower (longer period; lower spatial frequency) than the gridstructure with pitch A. Embodiments wherein the apertures are arrangedin a periodic manner in only one of the first direction and the seconddirection may also be envisaged. By a suitable etching process, all theexposed portions of the grid elements 510, 512, 514, 516, 518, 520 areremoved. After the cut mask 522 is removed, we see at (c) the functionaldevice pattern which comprises the grid elements, separated by cuts orgaps. This device pattern may be the finished product structure, or someintermediate structure to which further steps are applied to produce thefinished product based on this pattern.

For the purposes of this example, only one processing step has beenshown in FIG. 5. In practice, further processes, including applicationof further grid elements, may be carried out to form a functional devicestructure in accordance with a particular pattern.

Referring to FIG. 6, a method is shown for forming the metrology target30 shown in FIG. 3 by the process described with reference to FIG. 5. Asdescribed above, the target structure is formed of a first targetstructure 31 and a second target structure 32. Each target structurecomprises features defined by a first lithographic step and featuresdefined by a second lithographic step. In the present example, formationof the first target structure begins at (a) with a first grid structure610 and the second target structure begins with a second grid structure612. These features comprise a grid structure that is comprised of aperiodic array of grid elements 614 spaced with a pitch A in the first(e.g. X) direction. The grid elements are arranged in a periodicarrangement in the first direction with a pitch A that is similar oridentical to that of corresponding product structures on the samesubstrate. Each grid element in this example comprises a linear elementwhich extends in the second (Y) direction.

The grid structures 610 and 612 are shown and labelled as distinctstructures purely for explanation. In a practical embodiment, a singlegrid structure may extend uniformly throughout both metrology targetareas, and also though product areas (402) where present. (Thedifference between product areas and metrology target areas in such acase is made in a second lithographic step, as described below.) It is,of course, to be noted that this value is exemplary only, and that anysuitable value can be chosen for the pitch A. Typically, the pitchshould match the pitch of the product features, so that ultimately anymeasured parameter relates accurately to what is achieved in the realproduct. In one example, the pitch is Λ=40 nm. The pitch is severaltimes smaller than the wavelength of radiation used in a typicalscatterometer, which may be in the range for example 400-700 nm. Thetechniques described herein may be use where the pitch of an underlyingperiodic structure is less than a fifth, or less than a tenth of thewavelength of the radiation used in a measurement.

Subsequently, during the second lithographic step and suitableprocessing modifications to the grid structure are made to form theproduct structure (if present on the same substrate) and the first andsecond target structures 31, 32.

To form the first target structure 31, a first cut mask 616 is formedusing the second patterning device MA2 in the second lithographic step,as shown at (b). This cut mask comprises a plurality of apertures 620,the apertures in this example being arranged in a periodic manner inboth the first and second directions. In the present example, theapertures are illustrated as rectangular, although it is, of course, tobe appreciated that the apertures could be any suitable shape, and maybe distorted when produced in the real process. The apertures 620 of thefirst cut mask 616 are arranged on the cut mask so as to be offset fromthe grid elements by a known amount (also known as “bias”). In thepresent example, the apertures of the first cut mask are biased byΛ/2+d, (d<<Λ), that is to say an amount close to half the pitch of thegrid structure. The apertures 620 are therefore positioned so that eachaperture results in a partial cut being made to one or both of theadjacent grid elements 614, rather than neatly cutting one grid elementas they do in the product structure of FIG. 5. In the example with Λ=40nm, for example, one might choose d=5 nm. It should be noted that thespecific value for d is exemplary only, and that any suitable value ford could be chosen.

By setting the bias close to Λ/2 asymmetry in the target structure 31 ismade more pronounced, and more sensitive to any further misplacement ofthe cut mask apertures, such as would be caused by overlay error. Thisin turn increases the asymmetry of radiation scattered by the firsttarget structure and the sensitivity of that asymmetry to misplacementcaused by overlay error.

Similarly, to form the second target structure 32, the second patterningdevice MA2 and the second lithographic step are used to form a secondcut mask 618. The second cut mask comprises a plurality of apertures 620in a similar arrangement to that of the first cut mask. The apertures620 of the second cut mask 618 are biased by a different amount, stillclose to a half pitch when compared to their position in product areas(FIG. 5). In an example, the second target structure is formed with biasamount of Λ/2−d. The values for the pitch A and d are identical to thoseof the first cut mask, so that the bias amounts in the two targetstructures are spaced equally either side of a half pitch.

After completion of the etching and other process steps, the portions ofthe grid elements 614 that were exposed by the apertures have beenremoved, which results in the structures shown at (c). As can be seen,the second target structure in this example is a mirror image of thefirst target structure. This is only in the case where the bias amountsare spaced equally either side of the half pitch Λ/2, and in the casewhere overlay error between the first and second lithographic steps iszero. In a real target, with non-zero overlay error, the first andsecond target structures will not be mirror images of one another, andwill exhibit different degrees of asymmetry within themselves. Notethat, while the target structures 31, 32 are very different the productstructure and more sensitive to the parameter of interest (such asoverlay), they are formed by the identical steps and processing, and byidentical patterns in the patterning devices MA1, MA2 as the productstructures. Only the bias in their position relative to the underlyinggrid elements 614 is changed. In this way, the performance of thelithographic apparatus and other process steps when forming themetrology target structures should be the same as that when forming theproduct structures.

Referring now to FIG. 7, a method of measuring a parameter of alithographic process 700 according to an embodiment of the presentdisclosure will now be described. In step 701, a first target structureand a second target structure is provided on a substrate. In the presentembodiment, both target structures are formed by a process as describedabove with reference to FIG. 6. Of course they may be formed by whateverlithographic process is under investigation.

In step 702, a first angle-resolved scatter spectrum radiation isobtained. In the present embodiment, an angle-resolved scatterometer isused, as described with reference to FIG. 2 above. The first targetstructure is illuminated with a light source of selected polarizationand wavelength. The zeroth order light scattered by the first targetstructure is collected by the optical system of the scatterometer. Asexplained above, the detector is located in the back-projected pupilplane P (or alternatively in the conjugate pupil plane P′). The detector19 then captures a first scatter spectrum representing the angulardistribution of zeroth order light scattered by the first targetstructure. In the present example, 2-D scatter spectra are obtained. Inprinciple, only a 1-D scatter spectrum could be captured by thedetector, but the 2-D scatter spectrum contains more information inpractice, particularly as we are concerned in the present disclosurewith two-dimensional structures formed by multiple patterning.

In step 703, a second angle-resolved scatter spectrum is collected bythe detector in a similar fashion. The second target structure isilluminated with a light source. The zeroth order light scattered by thesecond target structure is collected by the optical system of thescatterometer. The detector then captures a second scatter spectrumrepresenting the angular distribution of zeroth order light scattered bythe second target structure.

As a preliminary step to steps 702 and 704, a process of selectingillumination conditions suitable for the specific target structures maybe performed.

In step 704, a measurement of a parameter of interest is derived from anasymmetry of the first and from an asymmetry of the secondangle-resolved scatter spectrum. In the present example, the parameterto be derived is overlay error, which is determined as described in thefollowing. In other examples, the parameter of interest may be exposuredose, focus or asymmetric lens aberration.

To measure asymmetry of the scatter spectrum, in one example aprocessing unit generates a first differential scatter spectrum bysubtracting from the first scatter spectrum a 180-degree rotated copy ofitself. The processing unit then generates a second differential scatterspectrum by subtracting from the second scatter spectrum an invertedcopy of itself. Asymmetries A_(Λ/2+d), for the first target structure,and A_(Λ/2−d), for the second target structure, are then determinedbased on the first and second differential scatter spectrums. In asimple example, the average pupil asymmetry is calculated simply bysubtracting the mean of all the pixel values in the left half of thedifferential scatter spectrum from the mean of all the pixel values inthe right half Alternative or more sophisticated asymmetry measures canbe envisaged, for example in order to maximize use of the availablesignal. Optionally, the average pupil asymmetry can be normalized to theoverall average intensity, as normalized asymmetry measurements are morecomparable with one another.

In FIG. 8 a curve 802 illustrates the relationship between overlay OVand asymmetry A in a conventional diffraction-based overlay measurement.The conventional overlay measurement will be described here purely asbackground. The curve normal represents asymmetry between +1^(st) and−1^(st) order diffraction signals. The idealized curve also assumes an‘ideal’, one-dimensional target structure having no offset and nostructural asymmetry within the individual structures forming the targetstructure. Consequently, the asymmetry of this ideal target structurecomprises only an overlay contribution due to misalignment of the firstfeatures and second features. This overlay contribution results from acombination of a known imposed bias amount and an (unknown) overlayerror. This graph is to illustrate the principles behind the disclosureonly, and the units of asymmetry A and overlay OV are arbitrary.Examples of actual dimensions will be given further below.

In the ‘idealized’ situation of FIG. 8, the curve 802 indicates that theintensity asymmetry A has a non-linear periodic relationship (forexample a sinusoidal relationship) with the overlay. The period A of thesinusoidal variation corresponds to the period or pitch A of the gridelements of the target structure, converted of course to an appropriatescale. The sinusoidal form is pure in this idealized example, but caninclude harmonics in real circumstances.

It is well known to the skilled person to use biased structures, such asgratings (having a known imposed overlay bias), to measure overlay,rather than relying on a single measurement. This bias has a known valuedefined in the patterning device (e.g. a reticle) from which it wasmade, that serves as an on-wafer calibration of the overlaycorresponding to the measured intensity asymmetry. In the drawing, thecalculation is illustrated graphically. As an example only, asymmetrymeasurements A_(+d) and A_(−d) are obtained for targets having imposedbiases +d an −d respectively. Fitting these measurements to thesinusoidal curve gives points 804 and 806 as shown. Knowing the biases,the true overlay error OV_(E) can be calculated. The pitch Λ of thesinusoidal curve is known from the design of the target structure. Thevertical scale of the curve 802 is not known to start with, but is anunknown factor which can be referred to as a 1^(st) harmonicproportionality constant, K₁. This constant K₁ is a measure of thesensitivity of the intensity asymmetry measurements to the targetstructure.

In mathematical terms, the relationship between overlay error OV_(E) andintensity asymmetry A is assumed to be:

A _(±d) =K ₁ sin(OV _(E) ±d)  (1)

where overlay error OV_(E) is expressed on a scale such that the targetpitch Λ corresponds to an angle 2π radians. A_(+d) and A_(−d) representasymmetry of the target structures with biases +d and −d respectively.Using two measurements of targets with different, known biases (e.g. +dand −d) the overlay error OV_(E) can be calculated without knowing K₁using the relationship:

$\begin{matrix}{{OV}_{E} = {{atan}( {\frac{A_{+ d} + {A_{-}}_{d}}{A_{+ d} - A_{- d}} \cdot {\tan (d)}} )}} & (2)\end{matrix}$

In the present disclosure, it is proposed to use bias amounts close to ahalf pitch, for example bias amounts Λ/2+d and Λ/2−d. The sameprinciples apply as in equations (1) and (2), except that the slope ofthe sinusoidal function will be opposite. On the other hand, in thepresent disclosure it is also proposed to use only zeroth order scatterspectra. The structures under investigation are periodic with periodsmuch shorter than the wavelength λ, of the illuminating radiation. Theperiod A may be for example less than 0.2λ, or less than 0.1λ, and tocollect higher order diffracted radiation may be impossible with theavailable optical system. Strong asymmetry signals with the sinusoidalform shown in FIG. 8 are therefore not expected. The inventors haverecognized that, using bias amounts close to Λ/2 instead of bias amountsclose to zero can give useful asymmetry signals even in the zeroth orderscatter spectrum, for 2-D product structures formed by multiplepatterning in a single material layer.

Note that the pitch Λ in this context is not necessarily a periodicityof the finished 2-D target structure, but rather by the pitch of thegrid formed in the first lithographic step. This will be the shortestperiod of several periodic components present in the overall 2-Dperiodic structure after the second lithographic step. While it issensible for various reasons to use two bias values either side of Λ/2,and to have them equally spaced either side of Λ/2, this is not anessential requirement. One of the bias amounts could be exactly Λ/2, ifdesired; both could even be on the same side of Λ/2. The calculationscan be adapted to any pair of bias values close to Λ/2. Considering whatis “close to” a half pitch, this is a matter of choice andexperimentation for each target. In a practical implementation, it maybe desired to operate in a relatively narrow region of the sinusoidalfunction in Equation (1), so that the variation of asymmetry withoverlay can be considered to be linear. Not only the bias amounts shouldbe considered when identifying the operating region, but also theanticipated range of overlay error that will be added to the programmedbias in a real target structure. The bias amounts Λ/2±d for example maylie between 0.3Λ and 0.7Λ, or between 0.4Λ and 0.6Λ. In a particularexample, the parameter d is chosen to be d<Λ/4. In general, the exactsize of d may be optimized dependent on situational requirements andcircumstances. Larger values of d may be used to improve thesignal-to-noise ratio, and smaller values of d may be used to increasethe accuracy of the overlay calculation.

In practice, intensity asymmetry measurements are not only dependent onthe properties of the target structures, but are also dependent on theproperties of the light incident on the target structures.

FIG. 9 shows a number of exemplary simulated results for an exemplarytarget structure, each simulation having been performed using light witha particular wavelength. Each graph shows the normalized averageasymmetry for a measurement target with target structures as describedwith reference to FIG. 6. Overlay is plotted over the horizontal axis,while an amplitude of the asymmetry signal is plotted on the verticalaxis. In each case, the average pupil asymmetry has been normalized tothe overall average intensity. The wavelength of radiation used in eachmeasurement is shown by a label, ranging from 425 nm at top left,through to 700 nm at bottom. In the example simulated here, the firstfeatures of the first target structure and the third features of thesecond target structure both have a pitch Λ=40 nm. The third features ofthe first target structure may be biased for example by Λ/2+d, where d=5nm. The fourth features of the second target structure are biased byΛ/2−d.

In the present example, the asymmetry varies dependent on the wavelengthused in the measurement. By selecting the wavelength of the light used,it is possible to maximize the accuracy of the measurement. Differentwavelengths and polarizations may be more successful for a differentprocesses and different target designs. In the illustration of FIG. 9,TM polarization is chosen for all graphs, but polarization is aparameter of illumination that can be varied if desired.

Measurements can be taken at more than one wavelength and/orpolarization, if desired, for further improving accuracy of themeasurement. Results from different wavelengths can be combined in anysuitable manner, either before or after conversion to overlay values.Note that it may be desirable to optimize not only the amplitude of theasymmetry signal (vertical scale in FIG. 9 graphs), but also thelinearity of the curve. The wavelength(s) selected should be one (ormore than one) for which a strong signal is obtained which is more orless linear over the range of values expected for overlay (or otherparameter of interest). Among the examples illustrated in FIG. 9, simpleinspection can be used to select the best one for a given targetstructure.

FIG. 10 shows a number of exemplary simulated pupil images for one ofthe radiation wavelengths shown in FIG. 9. Each image shows thesimulated pupil image for a given value of overlay. The radiationwavelength and the overlay amount is shown by a label above each image.As can be seen, the wavelength of the radiation used is 425 nm and theoverlay values range from −6 nm to +6 nm.

The dimensions of the modern product structures are so small that theycannot be imaged by optical metrology techniques. Small featuresinclude, for example, those formed by multiple patterning processes, andpitch-multiplication (terms explained further above). In effect, thestructures are too small for traditional metrology techniques whichcannot “see” them. Hence, targets used for high-volume metrology oftenuse features that are much larger than the products whose overlay errorsor critical dimensions are the property of interest.

While scanning electron microscopes are able to resolve modern productsstructures, measurements performed with scanning electron microscopesare much more time consuming, as well as more expensive, than opticalmeasurements and result in the destruction of the measured wafer.

The inventors have recognized that it is possible to perform metrologymeasurements on structures with dimensions and processing similar toproduct structures, or on structures formed from product structures, byusing zeroth order light scattered by these structures. Furthermore, itwas recognized that using asymmetrical contribution of the measuredspectrum, weighted by a careful choice of weighting coefficients, it ispossible to determine an overlay error between two steps ofmulti-patterning process, for example.

In an aspect, there is provided a method of measuring a parameter of alithographic process comprising; illuminating a target structure withradiation wherein the target structure is formed by said lithographicprocess, obtaining an angle-resolved scatter spectrum of the targetstructure; and deriving a measurement of said parameter using asymmetryfound in the scatter spectrum of the target structure.

In some embodiments, using the asymmetry found in the scatter spectrumof the target structure comprises using regions of the scatter spectrumwhich are being equally spaced from a reference.

In some embodiments, the contribution of the asymmetry, found in thescatter spectrum of the target structure used in deriving of theparameter of the lithographic process, is modified by a weightingcoefficient.

Further, the illumination of the metrology apparatus described in FIG.2, can be directed towards regions of the wafer W containing a patternforming product structures. In usual experimental conditions, whenforming a product as a result of exposure of at least two patterningdevices, it is expected that an overlay error will appear, overlay errorbeing between said formed pattern structures. As a way of an example,the two patterns can be formed by a patterning device corresponding to aperiodic line structure and a patterning device corresponding to a cutmask.

The target structure, formed by product structures as described in theparagraph above, or formed by structures similar to product structures,is illuminated with a light source of selected polarization andwavelength. The zeroth order light scattered by the target structure iscollected by the optical system of the scatterometer. As explainedabove, the detector is located in the back-projected pupil plane P (oralternatively in the conjugate pupil plane P′). The detector 19 thencaptures a first scatter spectrum representing the angular distributionof zeroth order light scattered by the first target structure. In thepresent example, 2-D scatter spectra are obtained. In principle, only a1-D scatter spectrum could be captured by the detector, but the 2-Dscatter spectrum contains more information in practice, particularly aswe are concerned in the present disclosure with two-dimensionalstructures formed by multiple patterning. Therefore, a parameter of alithographic process is measured with a method comprising: illuminatinga target structure with radiation wherein the target structure is formedby said lithographic process, obtaining an angle-resolved scatterspectrum of the target structure; and deriving a measurement of saidparameter using asymmetry found in the scatter spectrum of the targetstructure.

Further, the method is using regions of the scatter spectrum which arebeing equally spaced from a reference when using the asymmetry found inthe scatter spectrum of the target structure. For example, when themeasured spectrum is a 2-D scatter spectrum, measured in the pupil planeP, the reference can be one of the two axes of 2-D coordinate system. Inthis case, the reference can be the x-axis or the y-axis. It should berecognized that the x-axis and the y-axis form symmetry references as aline. Further, in the same 2-D coordinate system, the origin of thecoordinate system can be considered a reference as well. In this case,the reference would be a point.

The step of deriving a measurement of a parameter of a lithographicprocess uses regions of the 2-D measured spectrum, as found in the pupilP, regions which are symmetrical with respect to a reference. Bysubtracting said symmetrical contribution of the pupil, one is able tofind an indication of a degree of asymmetry present in the measured 2-Dspectrum. The step of subtracting said symmetrical portions of the pupilforms a characteristic SS. The asymmetry of the 2-D spectrum iscorrelated to the overlay error between patterned structures formed indifferent lithographic steps, for example. The regions which are used inderiving the overlay error can be single pixels or can be groups ofpixels, said groups having an internal symmetry, or said groups havingno symmetry at all.

Further, the contribution of the asymmetry, found in the scatterspectrum of the target structure used in deriving of the parameter ofthe lithographic process, is modified by weighting coefficients. Eachcharacteristic SS will represent an indication of the asymmetry of the2-D spectrum as measured in the pupil and an indication of the symmetryof the asymmetry as present in the 2-D scatter spectrum. It isrecognized that one can measure multiple SS characteristics, each one ofthe SS characteristics being a contribution from a different region ofthe measured 2-D spectrum or obtained by using different weightingcoefficients. By introducing a weighting factor for each characteristicSS, it is possible to enhance detection of the overlay error.

In an embodiment, the weighting coefficient is obtained from theasymmetric Jacobian part of the symmetric position. The region in the2-D spectrum sensitive to the overlay error can be obtained by computingthe Jacobian for the nominal target model of the target structure usingan analytical or computational method, such as, for example, RCWA. Theweighting coefficients follow from the asymmetric part of this Jacobian.

In an embodiment, the weighting coefficient is obtained from theasymmetric Jacobian calculated at different overlay errors. Due tonon-linear 2-D spectrum responses, the Jacobian may change when thetarget structure changes due to process variations. The asymmetric partof a (weighted) Jacobian average, obtained from models of the targetstructure corresponding to different process variations, can be used tomake the overlay measurement more robust to process variations.

In an embodiment, the weighting coefficient is obtained from a Design ofExperiment (DoE). A DoE can be used to determine the region in themeasured 2-D spectra that is sensitive to the overlay error, by, e.g.,applying a PCA (principal component analysis) to the asymmetry of thesemeasured 2-D spectra. The weighting scheme directly follows from one ormore obtained principal components.

Although patterning devices in the form of a physical reticle have beendescribed, the term “patterning device” in this application alsoincludes a data product conveying a pattern in digital form, for exampleto be used in conjunction with a programmable patterning device.

Further embodiments according to the invention are provided in belownumbered clauses:

1. A method of measuring a parameter of a lithographic process, thelithographic process being for forming a two-dimensional, periodicproduct structure in a single material layer using two or morelithographic steps, the method comprising:

providing first and second target structures, each target structurecomprising a two-dimensional periodic structure formed in a singlematerial layer on a substrate using first and second lithographic steps,wherein, in the first target structure, features defined in the secondlithographic step are displaced relative to features defined in thefirst lithographic step by a first bias amount that is close to one halfof a spatial period of the features formed in the first lithographicstep, and, in the second target structure, features defined in thesecond lithographic step are displaced relative to features defined inthe first lithographic step by a second bias amount close to one half ofsaid spatial period and different to the first bias amount;

obtaining an angle-resolved scatter spectrum of the first targetstructure and an angle-resolved scatter spectrum of the second targetstructure; and

deriving a measurement of said parameter using asymmetry found in thescatter spectrum of the first target structure and asymmetry found inthe scatter spectrum of the second target structure.

2. A method according to clause 1, wherein obtaining the angle-resolvedscatter spectrum of each target structure comprises:

illuminating the target structure with radiation; and

detecting the angle-resolved scatter spectrum using zero order radiationscattered by the target structure.

3. A method according to clause 1 or 2, wherein the spatial period ofeach target structure is significantly shorter than a wavelength of theradiation used to illuminate the target structures.

4. A method according to clause 2 or 3, further comprising selecting thewavelength of radiation from a range of available wavelengths so as tooptimize strength and linearity of asymmetry in the angle-resolvedscatter spectra of the target structures.

5. A method according to any preceding clause, wherein the step ofderiving said parameter comprises calculating a measurement of overlayerror relating to said product structures using the asymmetry found inthe scatter spectrum of the first target structure, the asymmetry foundin the scatter spectrum of the second target structure and knowledge ofthe first bias amount and the second bias amount.

6. A method according to any preceding clause wherein features of saidtarget structures that are defined in the first lithographic stepcomprise a grid structure defining said spatial period in a firstdirection, and features of said target structures that are defined inthe second lithographic step comprise modifications of the gridstructure at locations spaced periodically in a two-dimensional periodicarrangement.

7. A method according to any preceding clause wherein the features ofsaid target structures that are defined in the first lithographic stepcomprise a grid structure defining said spatial period in a firstdirection, and features of said target structures that are defined inthe second lithographic step comprise cuts in elements of the gridstructure.

8. A method according to any preceding clause, wherein the first targetstructure and the second target structure have been formed by etchingand/or deposition processes after the first and second lithographicsteps have been used to define their features.

9. A method according to any preceding clause, wherein a productstructure has been formed in the same material layer elsewhere on thesame substrate using said first and second lithographic steps, andwherein, in the product structure, features defined in the secondlithographic step are not displaced relative to features defined in thefirst lithographic step by any bias amount.

10. A substrate for use in measuring a parameter of a lithographicprocess, the substrate comprising first and second target structures,each target structure comprising a two-dimensional periodic structureformed in a single material layer using said first and secondlithographic steps, wherein,

in the first target structure, features defined in the secondlithographic step are displaced relative to features defined in thefirst lithographic step by a first bias amount that is close to one halfof a spatial period of the features formed in the first lithographicstep, and,

in the second target structure, features defined in the secondlithographic step are displaced relative to features defined in thefirst lithographic step by a second bias amount that is close to onehalf of said spatial period and different to the first bias amount.

11. A substrate according to any preceding clause wherein features ofsaid target structures that are defined in the first lithographic stepcomprise a grid structure defining said spatial period in a firstdirection, and features of said target structures that are defined inthe second lithographic step comprise modifications of the gridstructure at locations spaced periodically in a two-dimensional periodicarrangement.

12. A substrate according to any preceding clause wherein the featuresof said target structures that are defined in the first lithographicstep comprise a grid structure defining said spatial period in a firstdirection, and features of said target structures that are defined inthe second lithographic step comprise cuts in elements of the gridstructure.

13. A substrate according to any preceding clause, wherein the firsttarget structure and the second target structure have been formed byetching and/or deposition processes after the first and secondlithographic steps have been used to define their features.

14. A substrate according to any of clauses 10 to 13, wherein a productstructure has been formed in the same material layer elsewhere on thesame substrate using said first and second lithographic steps, andwherein, in the product structure, features defined in the secondlithographic step are not displaced relative to features defined in thefirst lithographic step by any bias amount.

15. A set of patterning devices adapted for defining features of thefirst and second target structures in a lithographic process for themanufacture of a substrate according to any of clauses 10 to 14, the setof patterning devices including a first patterning device for use insaid first lithographic step and a second patterning device for use insaid second lithographic step to form said first and second targetstructures in said material layer.

16. A set of patterning devices according to clause 15, wherein saidpatterning devices are further adapted for defining features of aproduct structure in the same material layer elsewhere on the samesubstrate using said first and second lithographic steps, and wherein,in the product structure, features defined in the second lithographicstep are not displaced relative to features defined in the firstlithographic step by any bias amount.

17. A metrology apparatus arranged to perform the method of any ofclauses 1 to 9.

18. A metrology apparatus according to clause 17, further comprising:

a support for a substrate on which a first target structure and a secondtarget structure have been formed;

an optical system for selectively illuminating each target structurewith radiation and collecting at least zero order radiation scattered bythe target structure;

a detector for detecting an angle-resolved scatter spectrum of eachusing said zero order radiation; and

a processor arranged to derive a parameter of a lithographic processusing asymmetry of the angle-resolved scatter spectrum of the firsttarget structure and asymmetry of the angle-resolved scatter spectrum ofthe second target structure.

19. A lithographic system comprising:

a lithographic apparatus for use in a lithographic process; and

a metrology apparatus according to clause 17 or 18 for use in measuringa parameter of the lithographic process using first and second targetstructures formed at least partially using the lithographic apparatus.

20. A computer program product comprising machine readable instructionswhich, when run on a suitable processor, cause the processor to performthe deriving step of the method of any of clauses 1 to 9.

21. A computer program product according to clause 20, furthercomprising machine readable instructions for controlling a metrologyapparatus to illuminate said first and second target structures withradiation and to detect said angle-resolved scatter spectra for use inthe deriving step.

22. A method to determine an overlay error on a substrate on whichproduct structures have been formed, the product structures includingfirst product features that have been defined by a first lithographicprocess and second product features that have been defined by a secondlithographic process, the overlay error comprising a positionaldeviation between the first product features and the second productfeatures, the method comprising:

providing a first target structure on the substrate, the first targetstructure comprising first target features defined by the firstlithographic process and second target features defined by the secondlithographic step, a positional relationship between the first targetfeatures and the second target features depending on a first bias valueand the overlay error; and

providing a second target structure on the substrate, the second targetstructure comprising third target features defined by the firstlithographic process and fourth target features defined by the secondlithographic step, a positional relationship between the third targetfeatures and the fourth target features depending on a second bias valueand the overlay error;

detecting a first angle-resolved scatter spectrum using zero orderradiation diffracted from the first target structure;

detecting a second angle-resolved scatter spectrum using zero orderradiation diffracted from the second target structure;

calculating a measurement of the overlay error based on asymmetryobserved in the first angle-resolved scatter spectrum and the secondangle-resolved scatter spectrum and on knowledge of the first bias valueand the second bias value.

23. A method of measuring a parameter of a lithographic processcomprising:

illuminating a target structure with radiation wherein the targetstructure is formed by said lithographic process,

obtaining an angle-resolved scatter spectrum of the target structure;and

deriving a measurement of said parameter using asymmetry found in thescatter spectrum of the target structure.

24. A method according to clause 23, wherein obtaining theangle-resolved scatter spectrum of the target structure comprisesdetecting zero order radiation scattered by the target structure.

25. A method according to the clause 23, wherein each target structurecomprising features forming a two-dimensional array.

26. A method according to the clause 23, wherein the target structurecomprises features having non-zero components in a complementary twodimensional Fourier space.

27. A method according to any preceding clause, wherein the step ofderiving said parameter comprises calculating a measurement of overlayerror relating to said product structures using the asymmetry found inthe scatter spectrum of the target structure.

28. A method according to the clause 24, wherein using the asymmetryfound in the scatter spectrum of the target structure comprises usingregions of the scatter spectrum which are being equally spaced from areference.

29. A method according to the clause 28, wherein the reference is aline.

30. A method according to the clause 28, wherein the reference is apoint.

31. A method according to the clause 28, wherein the contribution of theasymmetry, used in deriving of the parameter of the lithographicprocess, is modified by weighting coefficients.

32. A method according to the clause 31, wherein the weightingcoefficients are obtained from the asymmetric Jacobian part of thesymmetric position.

33. A method according to the clause 31, wherein the weightingcoefficient is obtained from the asymmetric Jacobian calculated atdifferent overlay errors.

34. A method according to the clause 31, wherein the weightingcoefficient is obtained from a Design of Experiment.

35. A metrology apparatus arranged to perform the method of any ofclauses 23 to 34.

36. A metrology apparatus according to clause 35, further comprising:

a support for a substrate on which the target structure has been formed;

an optical system for selectively illuminating each target structurewith radiation and collecting at least zero order radiation scattered bythe target structure;

a detector for detecting an angle-resolved scatter spectrum of eachusing said zero order radiation; and

a processor arranged to derive a parameter of a lithographic processusing asymmetry of the angle-resolved scatter spectrum of the targetstructure.

37. A lithographic system comprising:

a lithographic apparatus for use in a lithographic process; and

a metrology apparatus according to clause 35 or 36 for use in measuringa parameter of the lithographic process using target structures formedat least partially using the lithographic apparatus.

38. A computer program product comprising machine readable instructionswhich, when run on a suitable processor, cause the processor to performthe deriving step of the method of any of clauses 23 to 34.

39. A computer program product according to clause 38, furthercomprising machine readable instructions for controlling a metrologyapparatus to illuminate said first and second target structures withradiation and to detect said angle-resolved scatter spectra for use inthe deriving step.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography, atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used in relation to the lithographicapparatus encompass all types of electromagnetic radiation, includingultraviolet (UV) radiation (e.g., having a wavelength of or about 365,355, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation(e.g., having a wavelength in the range of 5-20 nm), as well as particlebeams, such as ion beams or electron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description by example, and not oflimitation, such that the terminology or phraseology of the presentspecification is to be interpreted by the skilled artisan in light ofthe teachings and guidance.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

1. A method of measuring a parameter of a lithographic process, thelithographic process being for forming a two-dimensional, periodicproduct structure in a single material layer using two or morelithographic steps, the method comprising: providing first and secondtarget structures, each target structure comprising a two-dimensionalperiodic structure formed in a single material layer on a substrateusing first and second lithographic steps, wherein, in the first targetstructure, features defined in the second lithographic step aredisplaced relative to features defined in the first lithographic step bya first bias amount that is close to one half of a spatial period of thefeatures formed in the first lithographic step, and, in the secondtarget structure, features defined in the second lithographic step aredisplaced relative to features defined in the first lithographic step bya second bias amount close to one half of the spatial period anddifferent to the first bias amount; obtaining an angle-resolved scatterspectrum of the first target structure and an angle-resolved scatterspectrum of the second target structure; and deriving a measurement ofthe parameter using asymmetry found in the scatter spectrum of the firsttarget structure and asymmetry found in the scatter spectrum of thesecond target structure.
 2. The method of claim 1, wherein the obtainingthe angle-resolved scatter spectrum of each target structure comprises:illuminating the target structure with radiation; and detecting theangle-resolved scatter spectrum using zero order radiation scattered bythe target structure.
 3. The method of claim 1, wherein the spatialperiod of each target structure is significantly shorter than awavelength of the radiation used to illuminate the target structures. 4.The method of claim 2, further comprising selecting the wavelength ofradiation from a range of available wavelengths so as to optimizestrength and linearity of asymmetry in the angle-resolved scatterspectra of the target structures.
 5. The method of claim 1, wherein thederiving the parameter comprises calculating a measurement of overlayerror relating to the product structures using the asymmetry found inthe scatter spectrum of the first target structure, the asymmetry foundin the scatter spectrum of the second target structure and knowledge ofthe first bias amount and the second bias amount.
 6. The method of claim1, wherein the features of the target structures that are defined in thefirst lithographic step comprise a grid structure defining the spatialperiod in a first direction, and features of the target structures thatare defined in the second lithographic step comprise modifications ofthe grid structure at locations spaced periodically in a two-dimensionalperiodic arrangement.
 7. The method of claim 1, wherein the features ofthe target structures that are defined in the first lithographic stepcomprise a grid structure defining the spatial period in a firstdirection, and features of the target structures that are defined in thesecond lithographic step comprise cuts in elements of the gridstructure.
 8. The method of claim 1, wherein the first target structureand the second target structure have been formed by etching and/ordeposition processes after the first and second lithographic steps havebeen used to define their features.
 9. The method of claim 1, wherein aproduct structure has been formed in the same material layer elsewhereon the same substrate using the first and second lithographic steps, andwherein, in the product structure, features defined in the secondlithographic step are not displaced relative to features defined in thefirst lithographic step by any bias amount.
 10. A substrate for use inmeasuring a parameter of a lithographic process, the substratecomprising: first and second target structures, each target structurecomprising a two-dimensional periodic structure formed in a singlematerial layer using the first and second lithographic steps, wherein:in the first target structure, features defined in the secondlithographic step are displaced relative to features defined in thefirst lithographic step by a first bias amount that is close to one halfof a spatial period of the features formed in the first lithographicstep, and, in the second target structure, features defined in thesecond lithographic step are displaced relative to features defined inthe first lithographic step by a second bias amount that is close to onehalf of the spatial period and different to the first bias amount. 11.The substrate of claim 10, wherein the features of the target structuresthat are defined in the first lithographic step comprise a gridstructure defining the spatial period in a first direction, and featuresof the target structures that are defined in the second lithographicstep comprise modifications of the grid structure at locations spacedperiodically in a two-dimensional periodic arrangement.
 12. Thesubstrate of claim 10, wherein the features of the target structuresthat are defined in the first lithographic step comprise a gridstructure defining the spatial period in a first direction, and featuresof the target structures that are defined in the second lithographicstep comprise cuts in elements of the grid structure.
 13. The substrateof claim 10, wherein the first target structure and the second targetstructure have been formed by etching and/or deposition processes afterthe first and second lithographic steps have been used to define theirfeatures.
 14. The substrate of claim 10, wherein a product structure hasbeen formed in the same material layer elsewhere on the same substrateusing the first and second lithographic steps, and wherein, in theproduct structure, features defined in the second lithographic step arenot displaced relative to features defined in the first lithographicstep by any bias amount.
 15. A set of patterning devices adapted fordefining features of the first and second target structures in alithographic process for the manufacture of a substrate, the set ofpatterning devices comprising: a first patterning device for use in thefirst lithographic step; and a second patterning device for use in thesecond lithographic step to form the first and second target structuresin the material layer.
 16. The set of patterning devices according toclaim 15, wherein: the patterning devices are further adapted fordefining features of a product structure in the same material layerelsewhere on the same substrate using the first and second lithographicsteps, and in the product structure, features defined in the secondlithographic step are not displaced relative to features defined in thefirst lithographic step by any bias amount.
 17. A metrology apparatusconfigured to perform the method of claim
 1. 18. A metrology apparatuscomprising: a support for a substrate on which first and second targetstructures, each target structure comprising a two-dimensional periodicstructure formed in a single material layer on a substrate using firstand second lithographic steps, wherein, in the first target structure,features defined in the second lithographic step are displaced relativeto features defined in the first lithographic step by a first biasamount that is close to one half of a spatial period of the featuresformed in the first lithographic step, and, in the second targetstructure, features defined in the second lithographic step aredisplaced relative to features defined in the first lithographic step bya second bias amount close to one half of the spatial period anddifferent to the first bias amount, have been formed; an optical systemconfigured to selectively illuminating each target structure withradiation and collecting at least zero order radiation scattered by thetarget structure; a detector configured to select an angle-resolvedscatter spectrum of each using the zero order radiation; and a processorarranged to derive a parameter of a lithographic process using asymmetryof the angle-resolved scatter spectrum of the first target structure andasymmetry of the angle-resolved scatter spectrum of the second targetstructure.
 19. A lithographic system comprising: a lithographicapparatus for use in a lithographic process; and a metrology systemcomprising: a support for a substrate on which first and second targetstructures, each target structure comprising a two-dimensional periodicstructure formed in a single material layer on a substrate using firstand second lithographic steps, wherein, in the first target structure,features defined in the second lithographic step are displaced relativeto features defined in the first lithographic step by a first biasamount that is close to one half of a spatial period of the featuresformed in the first lithographic step, and, in the second targetstructure, features defined in the second lithographic step aredisplaced relative to features defined in the first lithographic step bya second bias amount close to one half of the spatial period anddifferent to the first bias amount, have been formed; an optical systemconfigured to selectively illuminating each target structure withradiation and collecting at least zero order radiation scattered by thetarget structure; a detector configured to select an angle-resolvedscatter spectrum of each using the zero order radiation; and a processorarranged to derive a parameter of a lithographic process using asymmetryof the angle-resolved scatter spectrum of the first target structure andasymmetry of the angle-resolved scatter spectrum of the second targetstructure the metrology system configured to obtain a measurement of theparameter of the lithographic process using the first and second targetstructures formed at least partially using the lithographic apparatus.20. A method of measuring a parameter of a lithographic process, thelithographic process being for forming a two-dimensional, periodicproduct structure in a single material layer using two or morelithographic steps, the method comprising: providing first and secondtarget structures, each target structure comprising a two-dimensionalperiodic structure formed in a single material layer on a substrateusing first and second lithographic steps, wherein, in the first targetstructure, features defined in the second lithographic step aredisplaced relative to features defined in the first lithographic step bya first bias amount that is close to one half of a spatial period of thefeatures formed in the first lithographic step, and, in the secondtarget structure, features defined in the second lithographic step aredisplaced relative to features defined in the first lithographic step bya second bias amount close to one half of the spatial period anddifferent to the first bias amount; obtaining an angle-resolved scatterspectrum of the first target structure and an angle-resolved scatterspectrum of the second target structure; and a non-transitory computerprogram product comprising machine readable instructions which, when runon a suitable processor, cause the processor to deriving a measurementof the parameter using asymmetry found in the scatter spectrum of thefirst target structure and asymmetry found in the scatter spectrum ofthe second target structure.